It is already known to utilize vibrating masses as sensor elements for the gyros. These masses have most recently been constructed from quartz, which simplifies driving and sensing since quartz is piezoelectric. This also increases the possibility for constructing a robust gyro with high G numbers since the sensor can be made small. An example of such a sensor element is described in SE 89 00 666-2.
The base of this sensor element is firmly clamped, whereas the legs of the tuning fork can freely vibrate in two planes which are at right angles with respect to one another.
A mechanical vibration is generated piezoelectrically in one plane by means of drive electrodes. Vibrations are also produced, in a plane which is at right angles to the plane in which the drive electrodes generate vibration, through Coriolis forces which act on the sensor element when it is subjected to rotation around its longitudinal axis. These vibrations produced can be piezoelectrically sensed by means of sensor electrodes.
The legs of the sensor element have a transition part in connection to its base and an attachment part, which parts are rigid and do not participate in the vibration in any significant way. The drive and sensor electrodes cover significant parts of the vibrateable ends of the legs. Since the entire sensor element is integrally constructed of a piezoelectric material, for example quartz, all the material in the legs apart from the rigid transition part and mounting part will participate in the buildup of the respective sensing of the vibrations. This provides a gyro which withstands heavy environmental stresses, especially high acceleration stresses, and which has a small external volume. Due to weak currents and relatively high interference levels, however, it has been difficult to meet the necessary requirements for drift and zero signal.
Due to imperfections in production, there will always be a certain mechanical coupling between the two directions of vibration. This leads to an output signal being present from the sensor electrodes even if the sensor is not subjected to any rotation. Such "crosstalk" is undesired since it worsens the performance of the gyro. One way of eliminating this coupling between the two directions of vibration is described in the abovementioned which is namely to balance one of the legs in such a manner that the legs have identical vibration characteristics. This balancing can be done by attaching or removing a mass from suitably selected places on the legs. Even if balancing the masses of the legs is necessary for other reasons, it is desirable to be able to adjust the characteristics of the sensor element in another way since an adjustment of the characteristics of the sensor element by means of such masses is troublesome and increases the costs of sensor element production.
In the sensor element described in SE 89 00 666-2, both legs are utilized both for driving and sensing which implies that the two electrode sets must share the space on the legs. The electrodes must be optimized for the available space, and the sensor electrodes are preferably placed close to the transition between base and leg, while the drive electrodes are placed further out on the legs.
Such a placement of the electrodes presents problems, however, due to the fact that the weak currents which are generated on sensing can easily be affected via stray capacitances due to the high electrical voltages of the driving. It has therefore been difficult to achieve the desired drift and zero signal requirements with the type of electrode configurations described in the above patent.
There is a requirement for small inexpensive gyros which have a high G number. The disadvantages with small dimensions in a quartz sensor are, however, that very small output signals are obtained which are easily disturbed by external factors.
As the accuracy requirements increase relating to drift and zero signal, effects such as stray capacitances and unevenly etched geometry will acquire an increased negative significance. The capacitance arising between the drive electrodes, Co, also entails a drift (caused by, for example, temperature variations) since it frequently cannot be completely compensated.